1. Field of the Invention
The present invention pertains to generating electronic sound. More particularly, the present invention relates to a stereo sound circuit device for providing three-dimensional surrounding sound effect.
2. Description of Related Art
With the progress of electronic technology and the widely spread use of the multi-media, the three-dimensional surrounding sound can be implemented by enhancing the stereo sound effect. That is, with only a pair of speakers or earphones, the listener may hear the sound coming from various directions in the space. Therefore, the realistic feeling of the music can be increased. Furthermore, the realistic hearing enjoyment can be provided with the use of an electronic sound generating device, such as a multimedia computer or a virtual reality helmet.
Generally, the three-dimensional surrounding sound is implemented by the HRTF (Head Related Transfer Function) measurement. The HRTF refers to a process that a sound from a specific sound source transfers to the drum of the ear. Due to the resonance and diffraction effects of the pinna structure, shoulder and torso, a filtering effect is thus applied to the sound wave. Therefore, the HRTF at a specific position can be presented by a pair of impulse responses to the drums of two ears or microphones in two ears of a simulated head. The pair of impulse responses includes the frequency responses, the relative time differences, and the relative strength differences corresponding to the two ears.
In order to generate a three-dimensional sound effect, a digital filter with time delay must be made by software or hardware. As shown in FIG. 5, the signals of the left ear and right ear are respectively processed by a finite impulse response digital filter and a time delay element, so as to provide the HRTF effect and add the three-dimensional surrounding sound. However, although the added surrounding sound may generate a three-dimensional surrounding effect, the original sound may be distorted. Therefore, there is a demand to have a novel sound device capable of providing the three-dimensional surrounding sound effect, while the original sound is almost not distorted.